I think it’s hobbyists with metal detectors and they are required by law to report what they find. Maybe someone else knows for sure.
Cool to see, however.
When something is found, it must be reported. After it is declared treasure, a committee of experts on treasure decides the value. When a museum, usually the local, buys it, the money goes to whoever finds it. Minus income tax, of course. If found by detectorists, they have to split it 50-50 with the land owner.